The cabbie is driving out further and further into the middle of nowhere. We start off in Bishop Auckland, county Durham, and eventually get to a field with a duck pond. The cabbie tells me that this is a village and there are only two houses in it, and David Bellamy lives in the one on the right. I walk up the long drive. There is no obvious front door. There is a side door that could be a shed. No bell, no knocker, no people about. I walk in. There are numerous pairs of boots in a little porch, all sorts of illustrated birds and flowers on the wall, and a cartoon of Bellamy.

"Hello," I call.

"D'you mind dogs?" a woman shouts from behind another door. No, not at all, I say. She opens the door. A white lurcher, Lucy, bounds up and knocks me over. Lucy is followed by a Doberman, Remmy, who also knocks me over. Remmy is followed by a young blonde woman who introduces herself as Rosemary. I'm confused. I know Bellamy's wife is a biologist called Rosemary, but I also know that they have been married for the best part of 50 years.

Rosemary is followed by Bellamy. He's a huge man, all over - face, body, beard, laugh. He's wearing white trousers, a blue shirt and no socks or shoes. His toes are as large as most people's fingers. Bellamy has just written an entertaining and honest autobiography called The Jolly Green Giant. It's an apt title. He is all three of them. But the jollyness is only one side. Like so many idealists, there is a darker side.

In the 70s and 80s the botanist and academic became one of the most-loved faces on television. But he hasn't been on telly for years - Bellamy believes this is because he has been too outspoken.

There is the classic Bellamy image of him staring out of the foliage, eyes bulging, cheeks bursting with childlike enthusiasm. Kids loved him because he belonged to their world - a world of creepy crawlies, mudbaths, and the great outdoors. Everyone could do a Bellamy impersonation but Lenny Henry's "Gwapple me gwape nuts" was the most famous. I can't help myself, and do a full-throttle "gwapple me gwape nuts" in front of him. "That's it!" he says, "You've got it!" What does it mean? "I don't know. I never said that."

He still speaks in that strangulated drawl - part muffle, part spit, part beard. I catch about half his lava-flow of words. He apologises. "I do mumble a bit, don't I?" A bit? He roars with laughter and promises to speak slowly and clearly. Some hope.

He leads me and the dogs into a room that was a garage until last year, and is now furnished in beautiful Fair Trade woods from Mexico and Africa. He offers a spatter of local history as he walks. "This was the Bishop of Durham's ironworks in the 15th century, and the pond in the garden dates from 1408."

Bellamy last made big news in 1997 when he stood against John Major for the anti-European Referendum party. He sounded every bit the Little Englander, as he denounced a "federal Germany". I assumed he had become - or always had been - a rabid rightwinger. Today, he's railing against all forms of political correctness - language, race, politics in general.

A beautiful black woman walks in with coffee and biscuits. "That's my middle daughter, Brighid." Bellamy has five children - four are adopted. Brighid's ancestry is Guyanan, Eoghain's is Kashmiri and Karen's is Afro-Caribbean, Henrietta's is English. Rosemary had five miscarriages after their first child, Rufus, but they had always believed the world was over-populated and planned to adopt anyway. So they just adopted more than planned.

He tells me that Brighid, like Bellamy himself, had hoped to be a ballet dancer. "She worked bloody hard at it for years, until her knees collapsed." As a 14st 7lbs 14-year-old, Bellamy's dreams were crushed from the start.

Biodiversity has always been his big thing, and his family became a model of all that he believed in. "We used to sit around the table and discuss the world. I had the United Nations there, and they're still bloody well united. They really are siblings. They argue with each other, they'd die for each other."

So has he become a raving rightwinger? He roars again. "I am a socialist, but I don't have anyone to vote for. If Wedgy Benn was 40 years younger we might still have a socialist party."

He tells me how he got talking to a man on a train some time before the 1997 election and told him he was torn between voting for Arthur Scargill's Socialist Labour party and the Referendum party. The man told him he was an agent for the Referendum party, and soon after he got an invitation to lunch from Sir James Goldsmith, its founder. "It was the most fascinating lunch I've ever had. A bloody good lunch. I said: 'Why have you suddenly become a goody after being a baddy?' And he said: 'You go into a fish restaurant and the fish are full of poison, and that's why I'm dying of pancreatic cancer.' And I said, 'Well, I don't think you can prove that.' And he said, 'No, I can't prove it, but you say some bloody stupid things too.'"

It was then that Goldsmith asked him to stand against Major. "In some ways it was probably the most stupid thing I ever did because I'm sure that if I have been banned from television, that's why. I used to be on Blue Peter and all those things, regularly, and it all, pffffft, stopped."

Actually, he says, his TV career had stopped some time before that - he made his last BBC series eight years ago. Perhaps he is too scruffy for telly these days, I suggest. He gives me a look. "There are some bloody scruffy people on television these days. Very scruffy in what they think and the way they talk."

His grandson Theo, who lives with him, walks in. Theo is the son of Brighid, but he could be a biological Bellamy - he's 11, huge and full of laughter. "He's going to be a very good rugby player - look at his build," Bellamy says proudly. He says he loves children, always has. "You know, I really should have chucked the university up and gone and run an orphanage in Africa."

Bellamy's book is great on his own childhood in Surrey: from the kidney illness that should have killed him (he heard the hospital sister tell his mother, "He won't be with us tomorrow.") to ducking and diving in Doodlebug Alley when the bombs were falling in the war (two hit his house); to chatting to the ghost of his grandmother by a flowering yucca plant on his way home, before he knew she had died; to his ballet and rugby obsessions; to working at the local Paynes factory where the girls got up to all kinds of tricks with the boys and molten chocolate. He was such an innocent young man, had no idea that girls were interested in sex until he got to the factory. "It was much worse at the ink factory because they used to push it into an ink bottle and then amuse you. Eheh-heh-heh!" What d'you mean, I say, alarmed. "They stuck your willy in a bloody ink bottle, and then they titivate you so it got stuck in the ink bottle. Eheh-hehheh! That was painful."

Somehow that story makes me want to pee. On the way to the loo I meet another Rosemary. She is small and shy, gentle and elderly. "Ah, that's my wife. The other Rosemary works for us." He says he has been so lucky. "I fell in love when I was 19, and I'm still happily married to the same woman."

Suddenly he lurches into a monologue about watching the floods in Europe on television, which turns into a monologue on the bloody clutter of civilisation, which turns into a monologue about how Pokémon has turned Theo into an archivist par excellence , which turns into a monologue about his new consultancy, which turns into a surreal stream of consciousness about married bloody albatrosses. It's all hugely entertaining, but I haven't got a clue what he's going on about.

Your mind is amazing, I say - interested in so many things, and all over the place. "Well, that's the problem isn't it?" he says, quietly. He tells me his favourite painter is Hieronymous Bosch. "In one of his pictures there is a self-portrait of him looking down a glass telescope, and there is a rat sitting on the other end looking back. And that is me. I think I'm Hieronymous Bosch looking through a glass telescope because my education has never made me focus on anything, and the rat is saying you should be focusing."

In his autobiography he mentions, in passing, that although never diagnosed he believes he has a form of autism. Was he serious? "How much do you know about autism?" he asks, intensely. "Autism is not being able to relate. And I don't relate with large chunks of society. I can entertain them, but I can't relate to them. I can't find any way of ...w-w-what's the best example?" He stutters to a rare stop. "I mean I'm a very shy, withdrawn person in many ways and the person who comes out on television is me overcoming that."

He says he feels young at 69. "Wherever I go, people say, 'Why don't we see you on television these days?' And I say, 'Well, why don't you write to the BBC and ask them, I would like an answer.'" Has he asked? "I sort of have. But who do you write to at the BBC? Dear Greg Dyke, have you thrown me off the diddlydiddly ...?"

"You know," he says, with renewed vigour, "I got a call from a radio station the other day - they said, we saw a picture of you in the paper, and we thought you were dead."

There have been many disappointments in recent years. The book's subtitle is "The Autobiography of David J Bellamy OBE, Hon FLS, an Englishman". He says he considered removing the last bit. Why? "I don't know that I'm as proud of being an Englishman as I was when I was a little boy." When he was young he thought being English meant you stood up for the underdog, but he's not so sure now.

Bellamy is most disappointed by today's green groups. He rages against the failings of the pressure groups that have become corporates. "I said I'd never say anything that would harm the green movement, but we don't have a green movement now. We don't have campaigners campaigning for anything positive."

In 1983, Bellamy was jailed for blockading the Franklin River in protest at a proposed dam. "What the hell have Greenpeace and WWF done? They are paid very good salaries and they float around the world saying, 'We are helping the world,' but they haven't. WWF was set up 29 years ago to save the bloody panda and we can now breed pandas like the clappers, but there is no where to put them; all the forest has gone. Why the fuck didn't WWF buy all that forest? They could have had a wonderful thing: we'll back carbon sequestration tax and we'll rebuild the forests because bamboo takes up 20% more carbon than any other plant on this earth and makes 20% more oxygen. But, you see, once they've solved the problem they haven't got a job. That's what really worries me."

Does he ever fear that the pessimist in him has got the better of the optimist? "No, it can't. I've got four grandchildren. You've got to be optimistic." Anyway, he says, there are so many wonderful things in the world, so many things to do. Even if they did ask him back on telly he probably wouldn't have time, what with all the books he's working on, the global travelling, the campaigning, the ballet he's written, his diving, his schemes to put pensioners back to work.

Of course there are problems on the planet, he says, and yes, he does think we must act soon, but who would want a perfect world? "I was brought up as a strict Baptist and when I was about 14 I thought, I don't want to spend eternity sitting on a bloody cloud playing a harp. Utopia sounds good, but wouldn't it get bloody boring?"